Gas display panels of the type to which this invention relates have two flat glass plates that are spaced apart by a seal to contain an ionizable medium. A set of horizontally extending insulated conductors are located on one glass plate, and a set of vertically extending conductors are located on the other plate. When a suitable voltage is applied between one horizontal conductor and one vertical conductor, ionization occurs in a region at the cross over point of the two conductors and light is emitted. The cross over points and ionized regions inbetween are called cells, and a display pattern is formed by ionizing selected cells. However, upon application of the firing voltage the cell ionizes and emits light only briefly as free charges formed by the ionization migrate to the insulating glass walls of the cell where these charges produce an opposing voltage to the applied voltage and thereby extinguishes the ionization. The operation of initially ionizing a cell is called writing. Once a cell has been written a continuous sequence of light flashes can be produced by an alternating voltage called a "sustain" voltage. The amplitude of the sustain waveform can be made less than the amplitude required for the firing voltage, because the wall charges that remain from the preceding write or sustain operation produce a voltage that adds to the voltage of the sustain waveform to produce the ionizing voltage. A previously unwritten (or erased) cell is not ionized by the sustain waveform. In a gas panel of this type the sustain waveform is applied across all the horizontal conductors and all of the vertical conductors so that the gas panel maintains a previously written pattern of light emitting cells. The circuits that produce the sustain voltage are called "sustain circuits."
For a conventional write operation a suitable write voltage pulse is added to the sustain voltage waveform so that the combination of the write pulse and the sustain pulse produces ionization. In order to write an individual cell independently, each of the horizontal and vertical conductors has an individual selection circuit. Thus, applying a sustain waveform across all of the horizontal and vertical conductors but applying a write pulse across only one horizontal conductor and one vertical conductor will produce a write operation in only the one cell at the intersection of the selected horizontal and vertical conductors. An erase operation can be thought of as a write operation that proceeds only far enough to allow the previously charged cell walls to discharge, it is closely similar to the write operation except for timing and amplitude, and the circuits that produce both the write or erase pulses are called "write-erase circuits."
The selection circuit usually comprises a transistor switch for each horizontal conductor and each vertical conductor. The horizontal and vertical selection circuits connect the associated conductors to the horizontal or vertical sustain waveform and to a selected one of the two voltage levels of a write-erase pulse.
Gas display panels may be comprised of thousands of individual cells. Each one of these cells requires a minimum sustain voltage level in order to maintain a written condition. Also, each of these cells is limited to a maximum sustain voltage level above which an unselected cell would be written. The difference between the maximum sustain voltage level and the minimum sustain voltage level is called the sustain voltage "margin" or "window." The margin of an entire gas display panel is the difference between the largest minimum voltage level of any cell in the panel and the lowest maximum voltage level of any cell in the panel. Unfortunately, it has been found that the margin shifts upward or downward and often decreases in size as the panel ages.
The drift of a predetermined operating voltage setting due to the aging of the circuits that produce that voltage is a well known problem in electronic circuits. The circuits which produce the sustain voltage in a gas display panel are subject to aging and as a consequence the predetermined sustain voltage level changes as the circuit ages. If the operating voltage initially set in the middle of the margin tends to drift, and the minimum sustain voltage tends to drift in an opposite direction, the panel loses its useful life much quicker than it theoretically should. Thus, it is the object of this invention to extend the useful life of the panel by shifting the sustain voltage frequency as a function of sustain voltage amplitude.